Electric motors are one of the most widely used machines. The applications range from automobiles to earth movers, from aerospace to marine machinery, from home appliances to medical equipment. Recently, there have been increasing demands for motors with greater power density. For the purposes of this application, power density can be defined as the amount of power delivered either per unit weight or per unit volume, expressed respectively as W/kg or W/liter. One way to increase power density is to elevate the speed of a motor. As the speed of a motor increases, so does its overall power. As a result, there is a trend in machine designs toward using smaller electric motors operating at higher speeds. In some applications, this results in the speed of the motor being higher than the required speed of the driven member. Therefore, it is often deemed necessary to include a speed reduction unit between the motor and the driven member to reduce the speed of the motor to the required speed of the driven member. Although this results in an overall higher power density, the speed reduction unit still limits power density because of the additional weight and volume.
One solution to this is a so-called gear-head motor where a gear reduction unit is integrated with an electric motor. There are many types of gear-head motors including “precision” gear-head motors, which are capable of running at higher speeds and generally are much more expensive than “regular” gear head motors. However, even with precision-made gear heads, gear-head motors are often limited to operating speeds of 5,000 to 6,000 rpm. This has, to a large degree, prevented the gear-head motors from achieving their ultimate power-density potentials.
Recent developments in traction drives have demonstrated that a well-built traction drive can operate at higher speeds up to and exceeding 10,000 rpm and cost much less than gear-head drives. Thus, integrating a traction drive with an electric motor can increase the system power-density potential and thus extend the scope of application of electric motors.